Well, technically you could do it on one day, assuming the first tour got back on time and you could change in the lobby. You'll be in a wet bathing suit when you return from Dunn's but will want to wear dry capris or long shorts and tennis shoes for the zipline.
The Dunn's Tour is part of your package, so it's scheduled by CTI. It leaves at 9am and returns around noon.
The zipline is additional so you set up your own tour through the tour desk in the lobby. When we did this from CTI a few trips ago, there were 2 ziplining tours - one in the morning and one in the afternoon. We chose the afternoon because hubby dives so we had to leave around 12:30 and got back around 4:30.